Signal processing circuits are frequently used to read storage media and interpret obtained analog signals as discrete values stored on the media. Analog signal are often obtained from magnetic storage media. A transducer head may fly on a cushion of air over a magnetic disk surface. The transducer converts magnetic field variations into an analog electrical signal. The analog signal is amplified, converted to a digital signal and interpreted (e.g., using maximum likelihood techniques, such as using a Viterbi decoder). Tracking of stored data during a read operation is frequently performed using feedback or decision aided gain and timing control.
Increasing the amount of data stored on a magnetic medium can result in an increased error-rate unless error detection and correction techniques are used to compensate. If a read signal from a unit of storage (e.g., a cluster or a sector) on a disk cannot be interpreted adequately, typically the storage unit is read again one or more times to obtain a signal that can be interpreted, or to determine that there is a disk error.